C. E. Beecher — Origin and Significance of Spines. 135 



consolidated tergites of the second antenna] and mandibular seg- 

 ments of the Decapod Crustacea, by which the carapace has 

 been produced, has resulted in a constant pressure on the 

 dorsal arches of the succeeding five cephalic and five thoracic 

 segments, until as a result we have an atrophy of the dorsal 

 arches of as many as ten segments, these being covered by the 

 carapace." 



The restraint of the environment through unfavorable con- 

 ditions is the antithesis of A, or the influence of constructive 

 external stimuli, and is considered as the extrinsic operation of 

 destructive agencies. It is evident that external unfavorable 

 conditions will repress growth, with a resultant atrophy of the 

 structures affected. In this way, also, the environment may 

 cause the disuse of an organ, which by consequent suppression 

 may dwindle away to a spine, as in the leaves and branches of 

 desert plants, and the spurs of the Python 60 representing the 

 hind limbs. It may likewise repress growth, as in the spines 

 on the lower side of the poriferous coral, Michelinia favosa* 

 representing aborted attempts at budding, the failure being 

 due to the unfavorable position of the buds for securing food. 



The restraint of the environment may also act in a mechan- 

 ical manner to produce spines, as will be shown subsequently 

 in some Brachiopods and Trilobites. Furthermore, spines aris- 

 ing through any phase of external restraint, may secondarily 

 come under the influences of natural selection, and be useful 

 for protection and offense, or conform to other external 

 demands. 



Under the head of external restraint, therefore, are the fol- 

 lowing categories : 



C. From External Restraint. 



CI. — Restraint of environment causing suppression of struc- 

 tures. 



C2. — Mechanical restraint. 



C 3.— Disuse. 



C 4. — Secondarily for protection, offense, etc. 



D. Deficiency of Growth Force. 



The growth force in organisms may be reduced in several 

 ways, the most general and obvious modes being by an unfa- 

 vorable environment, lack of physiological plasticity, too close 

 interbreeding, pathologic influences, and parasitism. The first 

 commonly implies a scarcity of food, or it may be that the 

 temperature, moisture, light, elevation, or other conditions are 

 unsuitable to the normal development. The lack of physiolog- 

 ical plasticity affects growth force by its resistance to change, 

 and is most strongly apparent in highly specialized forms. The 



